Measurement of the differential cross section for top quark pair production in pp collisions at sqrt(s) = 8 TeV (1505.04480v2)

Abstract: The normalized differential cross section for top quark pair (tt-bar) production is measured in pp collisions at a centre-of-mass energy of 8 TeV at the CERN LHC using the CMS detector in data corresponding to an integrated luminosity of 19.7 inverse femtobarns. The measurements are performed in the lepton + jets (e/mu + jets) and in the dilepton (e+e-, mu+mu-, and e+-mu-+) decay channels. The tt-bar cross section is measured as a function of the kinematic properties of the charged leptons, the jets associated to b quarks, the top quarks, and the tt-bar system. The data are compared with several predictions from perturbative quantum chromodynamics up to approximate next-to-next-to-leading-order precision. No significant deviations are observed relative to the standard model predictions.

Measurement of the Differential Cross Section for Top Quark Pair Production at 8 TeV

This paper outlines the precise measurement of normalized differential cross sections for top quark pair production ($t\bar{t}$) in proton-proton (pp) collisions at a center-of-mass energy of 8 TeV, conducted using the Compact Muon Solenoid (CMS) detector at the CERN Large Hadron Collider (LHC). The paper leverages a dataset corresponding to an integrated luminosity of 19.7 fb$^{-1}$, significantly larger than that at 7 TeV, allowing for enhanced statistical precision and the reduction of systematic uncertainties.

Methodology

The analysis was performed in two decay channels: the lepton+jets ($e/\mu+jets$) and dilepton decay channels. Top quark pairs usually decay into a W boson and a bottom quark, and the paper focuses on W decays into electrons or muons and neutrinos.

To extract $t\bar{t}$ events, events with lepton+jets are selected based on the presence of one lepton, at least four jets (two of which are b-tagged), and significant missing transverse energy (indicating neutrinos). In dilepton channels, events consist of two opposite-charge leptons and at least two jets, with one being b-tagged. Backgrounds, primarily from single top quark production and W/Z+jets, are estimated using simulations and data-driven methods.

A kinematic reconstruction technique is employed to associate jets with b quarks and to determine the properties of the top quark pair system. The method was refined to improve the number of correctly reconstructed events and reduce bin migration effects in the cross-section measurements.

Results

The normalized cross sections are evaluated as functions of various kinematic properties:

• Lepton and b-jet properties: Transverse momentum ($p_T$) and pseudorapidity ($\eta$).
• Top quark and $t\bar{t}$ system properties: Transverse momentum, rapidity, and invariant mass.

These measurements are presented in a fiducial phase space defined to match experimental funding (particle-level results for leptons and b-jets) and extrapolated to all phase space for the top quark and $t\bar{t}$ system (parton-level results).

The results are compared to theoretical accurate Quantum Chromodynamic (QCD) predictions from several generators, including \textsc{MadGraph}+\textsc{Pythia}, \textsc{Powheg}+\textsc{Pythia}, and \textsc{Powheg}+\textsc{Herwig}, and with next-to-next-to-leading order (NNLO) and next-to-next-to-leading-logarithm (NNLL) corrections.

Findings and Discussions

In general, theoretical predictions describe the data well, although the observed transverse momentum distributions for $t\bar{t}$ products in the data are slightly softer than expected, especially compared to \textsc{MadGraph}+\textsc{Pythia} and \textsc{Powheg}+\textsc{Pythia}. The \textsc{Powheg}+\textsc{Herwig} combination provides a better overall description of the data, including the top quark $p_T$ and the $t\bar{t}$ invariant mass.

This analysis supports the robustness and precision of NNLO with NNLL calculations, offering valuable insights into the $\sqrt{s}=8$ TeV energy regime. The paper's results agree across different decay channels and with previous measurements at $\sqrt{s}=7$ TeV, reflecting consistency in $t\bar{t}$ production behavior across different energies.

Implications and Future Work

The paper enhances the understanding of $t\bar{t}$ production, a critical probe for testing the Standard Model (SM) and exploring potential new physics scenarios. Improved modeling and new data can further reduce the discrepancies observed in the $p_T$ spectrum. Future studies could focus on utilizing larger datasets and advancing reconstruction and simulation techniques. Moreover, results like these help refine PDFs essential for predicting $t\bar{t}$ production rates and distribution, aiding the development of more precise theoretical frameworks.